The present invention relates generally to initialization procedures for high speed bi-directional communication systems. More particularly, it relates to initialization procedures that are particularly well suited for communication systems employing multi-carrier modulation schemes.
With the increasing popularity of the Internet, video conferencing and other communications systems that require the transmissions of relatively large quantities of data to households and businesses, there have been corresponding demands for higher speed modems for use in bidirectional communications. Given the inherent limitations of single carrier modulation schemes, there has been an increasing interest in the use of multi-carrier modulation schemes. Some of the more popular systems contemplate the use of digital subscriber lines (e.g. telephone lines), cable lines, and various radio interfaces. In many of the proposed applications, point to multi-point transmission schemes are contemplated. By way of example, at the time of this writing, the Alliance For Telecommunications Industry Solutions (ATIS), which is a group accredited by the ANSI (American National Standard Institute) Standard Group is working on the next generation subscriber line based transmission system, which is referred to as the VDSL (Very High-Speed Digital Subscriber Line) standard. The VDSL standard is intended to facilitate transmission rates of up to 51.92 Mbit/s. Simultaneously, the Digital, Audio and Video Council (DAVIC) is working on a short range system, which is referred to as Fiber To The Curb (FTTC). A number of multi-carrier modulation schemes have been proposed for use in the VDSL and FTTC standards (hereinafter VDSL/FTTC). One proposed multi-carrier solution utilizes discrete multi-tone (DMT) signals in a system that is similar in nature to the ADSL standard that was recently adopted by ANSI for a slightly lower speed system. Other proposed modulation schemes include carrierless amplitude and phase modulated (CAP) signals; discrete wavelet multi-tone modulation (DWMr); and OFDM which is a simplified version of DMT.
A typical subscriber line based telecommunications local loop is illustrated in FIG. 1a. As seen therein, a unit 10 at a central location communicates with a remote units R1 over a discrete transmission line 18. Simultaneously, other units at the central location communicate with other remote units over different lines in the same cable. A variety of transmission media can be used as the transmission line. By way of example, coaxial cables, twisted pair phone lines, and hybrids that incorporate two or more different media all work well. This approach also works well in wireless systems. The remote units 22 may be end user units that may exist in a home, office or the like. Typically a number of remote units 22 are serviced by a particular central office. In currently installed systems, the remote units are often telephones, however, they may be fax lines, computer terminals, televisions or a wide variety of other devices capable of connecting to the xe2x80x9cphone linesxe2x80x9d. The central unit 10 may include a transceiver 32 for each line that is functionally broken into a transmitter 34 and a receiver 36.
In some embodiments, the central unit is a master server located at a central office that originates the communications. In other embodiments, the xe2x80x9ccentral unitxe2x80x9d may be a lower level distribution component in the system architecture that receives and retransmits signals. One embodiment of such a distribution component is illustrated in FIG. 1b. As seen therein, a trunk line 52 terminates at a distribution unit 54. In the embodiment shown, the trunk line takes the form of a fiber optic cable and the distribution unit takes the form of an optical network unit (ONU). The distribution unit 54 communicates with a plurality of remote units R1-RN over discrete lines 18, which again, may take the form of conventional twisted pair phone lines. As in the previously described embodiment, the remote units may be end user units that may exist in a home, office or the like. Typically a number of remote units are serviced by a particular ONU. By way of example, in North America, typical ONUs may service on the order of 4 to 96 remote units. In this embodiment, the ONU receives downstream source signals over one or more trunk lines and transmits the information embodied therein to the appropriate remote units as downstream communication signals. Similarly, the ONU receives upstream communication signals from the remotes and transmits the information embodied therein as upstream source signals. The source signals may be passed to a central office, another distribution unit or any other suitable location. A service provider would typically be arranged to provide the data to the central modem for transmission to the remote units 22 and to handle the data received by the central modem from the remote units. The service provider can take any suitable form. By way of example, the service provider can take the form of a network server. The network server can take the form of a dedicated computer or a distributed system.
The distance between the central unit 10, 54 and the furthest remote may vary a fair amount. By way of example, it is expected that in the VDSL/FTTC standards, twisted pair loop lengths of up to 1000 feet (300 meters) will be permitted for downstream communications at 51.92 MHz. Similarly, loop lengths of up to 3000 feet (900 meters) may be permitted for downstream communications at 25.96 MHz and loop lengths of up to 5000 feet (1500 meters) may be permitted for downstream communications at 12.97 MHz. As will be appreciated by those skilled in the art, shorter maximum loop lengths generally correspond to higher achievable data rates.
With any of the proposed multi-carrier modulation schemes, one problem that must be addressed is how to initialize a communication between the modems. Although a wide variety of initialization schemes have been proposed for multi-carrier communication systems, there are continued needs for improved initialization schemes.
In accordance with the purpose of the present invention, a variety of methods of initializing a connection between a remote modem and a central unit in a communication system that utilizes a multi-carrier modulation scheme are described.
In one aspect of the invention, a relatively long duration single frequency activation signal that ramps up and ramps down in intensity is utilized. In a preferred embodiment, the activation signal is transmitted on a sub-carrier that is outside a range of sub-channels used for data transmission in the multi-carrier modulation scheme. The activation signal may have a duration of a plurality of symbols, as for example a superframe.
In a separate aspect of the invention, a central synchronization signal is transmitted from the central unit to a remote unit. The central synchronization signal may be either in response to a remote activation signal, or initiated at the central unit. The remote unit returns a remote synchronization signal and the central unit replies with a central setup signal from the central unit to the remote unit in response to the remote synchronization signal. The remote then transmits a remote setup signal. The central unit is further arranged to transmit a central setup complete signal when it has completed a central setup procedure. The remote is also arranged to transmit a remote message after it has completed a remote setup procedure. However, the remote message is only sent after the remote unit has both completed the remote setup procedure and received the central setup complete signal. A central message is sent by the central unit after the remote message has been received by the central unit. The remote transmits a remote ready signal after the central message has been received and a central ready signal is sent from the central unit in response to the remote ready signal.
In one preferred embodiment, during a first superframe of each of the described signals, the transmitting unit sends only header symbols which constitute a first predetermined pattern in order to help clearly define a transition in signals. In some embodiments, in the event that an error is detected, the unit that detects the error transmits designated error symbols which constitute a second predetermined pattern.
In another aspect of the invention, a plurality of header symbols having a first predetermined pattern are transmitted during a first superframe of the central setup signal. A sequence of training symbol sets are then transmitted during a plurality of following superframes of the central setup signal. Further, during the transmission of the central setup complete signal, the sequence of training symbol sets continues to be retransmitted and a designated symbol in each superframe that is not to be used by the remote unit to obtain training information is set to a state indicative of the completion of the central setup procedure. With this arrangement a synchronous setup of the central unit and the remote unit is facilitated since the remote unit may continue to receive the training symbol set even after the central setup procedure has been completed.
In some embodiments, the central setup signal includes training signals. Sub-carrier line parameters are computed at the remote unit based at least in part on an analysis of the received training signals. The computed sub-carrier line parameters are then transmitted to the central unit as part of the remote message. The central unit then calculates a desired sub-carrier bit distribution based at least in part upon the received sub-carrier line parameters. The desired sub-carrier bit distribution is then transmitted to the remote unit as part of the central message.